1. Field of the Invention
The present invention relates to a direct-current stabilized power supply device that outputs a stabilized voltage.
2. Description of Related Art
Nowadays, direct-current stabilized power supply devices are widely used as power supply devices that can supply a stabilized voltage to a load regardless of variations in an input or a load or in surrounding environments. On the other hand, apparatuses that are provided with a digital circuit, such as computers or AV apparatuses, have been becoming increasingly popular in recent years, and such apparatuses are not able to function without a direct-current stabilized power supply device. Since these apparatuses are required to consume less energy for longer battery life and for less environmental impact, direct-current stabilized power supply devices with lower current consumption are sought after.
Used as the direct-current stabilized power supply device described above are a dropper-type stabilized power supply device that steps down an input voltage and then outputs it by using an output transistor as a type of variable resistance, and a chopper-type stabilized power supply device (a switching-type stabilized power supply device) that stabilizes an output voltage by controlling a duty ratio at which an output transistor is turned on/off.
Since the former dropper-type stabilized power supply device (the dropper regulator) stabilizes an output voltage by using the voltage drop across a transistor, it releases the voltage drop as heat. This makes the efficiency of this dropper-type stabilized power supply device not especially high when an input/output voltage difference is large. On the other hand, it offers ease of design and can find wide application because it suffers from less noise.
On the other hand, since the latter chopper-type stabilized power supply device (the chopper regulator) switches on/off an output transistor, thereby performing output control based on a duty ratio at which the output transistor is switched, it offers high efficiency when used in an application where an input/output voltage difference is large.
Incidentally, the stabilized power supply device has many functions such as overheat protection, overcurrent protection, and soft start, and has a built-in protection circuit for realizing the above described functions.
An example of a conventional dropper-type stabilized power supply device will be described with reference to FIG. 10. A conventional dropper-type stabilized power supply device 101 (hereinafter simply referred to as a “power supply device 101”) is built with an output transistor 102, a control circuit 104, and a constant voltage circuit 131 that feeds a voltage for driving the control circuit 104. The control circuit 104 is composed of a reference voltage source 126 that outputs a reference voltage Vref, an error amplifier 125, a drive transistor 133, an overheating protection circuit 118, an overcurrent protection circuit 119, an OR circuit 120, and a transistor 134.
An input voltage Vin outputted from the direct current power source 5 is fed to the emitter of the output transistor 102 and to the constant voltage circuit 131. The output of the direct current power source 5 is grounded via a capacitor 6. An error between a voltage obtained by dividing an output voltage Vout of the power supply device 101 with voltage dividing resistances 7 and 8 and the reference voltage Vref is amplified by the error amplifier 125. The error amplifier 125 controls a base current of the output transistor 102 via the drive transistor 133, whereby the output voltage Vout is kept at a constant level. A load 10 operates from the output voltage Vout. A terminal from which the output voltage Vout is outputted is grounded via the capacitor 9.
When abnormal events occur, the built-in protection functions provide protection for the power supply device 101. For example, the overheating protection circuit 118 prevents the junction temperature of the output transistor 102 from exceeding a certain level due to, for example, an increase in internal heat resulting from a heavy load or an abnormal increase in the ambient temperature by forcing the output transistor 102 to be turned off when the junction temperature reaches a certain level. On the other hand, the overcurrent protection circuit 119 protects the power supply device 101 from overcurrent by limiting an output current so that a current above a certain level does not flow therethrough.
When overheat protection or overcurrent protection is made to operate, a high level signal is fed from the overheating protection circuit 118 or the overcurrent protection circuit 119 to the OR circuit 120. This turns on the transistor 134, and then the base voltage of the drive transistor 133 takes a low level (for example, 0.1 V). As a result, the base current of the output transistor 102 is interrupted, turning off the output of the power supply device 101.
The constant voltage circuit 131 is a circuit that stabilizes the input voltage Vin by using, for example, a constant voltage diode so as to deliver a relatively constant voltage to the control circuit 104 as a supply voltage thereof. Here, assume that the input voltage Vin is 12 V, the output voltage of the constant voltage circuit 131 (that is, the supply voltage of the control circuit 104) is 2.7 V, and current consumption of the control circuit 104 is 10 mA. Then, electric power consumed for driving the control circuit 104 is 12 V×10 mA=120 mW.
Moreover, a regulator is disclosed in JP-A-2005-6442 (hereinafter referred to as Patent Publication 1) that interrupts the supply of electric power to a protection circuit when protection such as overheat protection is not needed.
As described above, in the power supply device 101 of FIG. 10, a relatively large electric power is consumed for driving the control circuit 104. On the other hand, in the regulator of Patent Publication 1, since the supply of electric power to the protection circuit is interrupted when protection is not needed, it can be expected to reduce the electric power consumption. However, this does not sufficiently contribute to the reduction of electric power consumption, because electric power consumed by a control circuit other than the protection circuit is not reduced.